The purpose of this study is to elucidate key aspects of the mechanism of root caries formation. Of particular importance is the direct determination of the cariogenic potential of dental plaque through the analysis of its extracellular aqueous phase, referred to as plaque fluid. The cariogenic potential of plaque is primarily influenced by the degree of saturation with respect to tooth mineral and levels of organic acids in plaque fluid, obtained following sucrose exposure. In addition, the mineral ion content of whole plaque appears also to be a caries determinant. In conjunction with the Clinical Core, these important factors will be assessed in an elderly population and correlated with the microbial composition of plaque, caries activity, and other clinical parameters. These studies will provide a direct measurement of the physical chemical factors involved in root caries initiation and a much- needed link between these factors, microbiological determinants, and caries activity. In addition, in vitro studies will be carried out to explain differences in the mechanism of caries formation in enamel and dentine. Total concentrations of inorganic and organic acid ions will be determined in plaque fluid obtained from exposed root surfaces from a large elderly population, before and after sucrose exposure. This data will be used to calculate the degree of saturation with respect to tooth mineral. The calcium, phosphate and fluoride contents of whole plaque will also be determined. The combined data will be correlated with other laboratory and clinical measurements from the same subject, as described within the Clinical Core. In addition, in vitro studies will be carried out to assess the relative influence of the following factors on dentine and enamel demineralization: salivary pellicle formation, the adsorption of relevant salivary proteins, inherent differences in ultrastructure and mineral properties, degree of saturation in experimental solutions, and fluoride ion concentrations in solution. In addition, the influence of several diphosphonates on dentine demineralization will also be studied, along with mechanistic aspects dentin remineralization. These combined studies will provide a unified picture of the caries process which may be useful in the design of anti-caries procedures.